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參數(shù)資料
| 型號： | ISL6525CB |
| 廠商： | INTERSIL CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Buck and Synchronous-Rectifier Pulse-Width Modulator (PWM) Controller |
| 中文描述： | SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PDSO14 |
| 封裝： | PLASTIC, MS-012-AB, SOIC-14 |
| 文件頁數(shù)： | 5/11頁 |
| 文件大?。?/td> | 120K |
| 代理商： | ISL6525CB |
5
PGOOD signal, not the falling edge. An internal hysteresis
guarantees glitch-free transition of PGOOD. Refer to
Programming PGOOD Delay Time section for more
information.
Functional Description
Initialization
The ISL6525 automatically initializes upon receipt of power.
Special sequencing of the input supplies is not necessary. The
Power-On Reset (POR) function continually monitors the input
supply voltages. The POR monitors the bias voltage at the VCC
pin and the input voltage (V
IN
) on the OCSET pin. The level on
OCSET is equal to V
IN
less a fixed voltage drop (see over-
current protection). The POR function initiates soft start
operation after both input supply voltages exceed their POR
thresholds. For operation with a single +12V power source, V
IN
and V
CC
are equivalent and the +12V power source must
exceed the rising VCC threshold before POR initiates operation.
Soft Start
The POR function initiates the soft start sequence. An internal
10
μ
A current source charges an external capacitor (C
SS
) on
the SS pin to 4V. Soft start clamps the error amplifier output
(COMP pin) and reference input (+ terminal of error amp) to
the SS pin voltage. Figure 3 shows the soft start interval with
C
SS
= 0.1
μ
F. Initially the clamp on the error amplifier (COMP
pin) controls the converter’s output voltage. At t
1
in Figure 3,
the SS voltage reaches the valley of the oscillator’s triangle
wave. The oscillator’s triangular waveform is compared to the
ramping error amplifier voltage. This generates PHASE
pulses of increasing width that charge the output capacitor(s).
This interval of increasing pulse width continues to t
2
. With
sufficient output voltage, the clamp on the reference input
controls the output voltage. This is the interval between t
2
and
t
3
in Figure 3. At t
3
the SS voltage exceeds the reference
voltage and the output voltage is in regulation. This method
provides a rapid and controlled output voltage rise.
Over-Current Protection
The over-current function protects the converter from a
shorted output by using the upper MOSFETs on-resistance,
r
DS(ON)
to monitor the current. This method enhances the
converter’s efficiency and reduces cost by eliminating a
current sensing resistor.
The over-current function cycles the soft-start function in a
hiccup mode to provide fault protection. A resistor (R
OCSET
)
programs the over-current trip level. An internal 200
μ
A (typical)
current sink develops a voltage across R
OCSET
that is
reference to V
IN
. When the voltage across the upper MOSFET
(also referenced to V
IN
) exceeds the voltage across R
OCSET
,
the over-current function initiates a soft-start sequence. The
soft-start function discharges C
SS
with a 10
μ
A current sink and
inhibits PWM operation. The soft-start function recharges C
SS
,
and PWM operation resumes with the error amplifier clamped
to the SS voltage. Should an overload occur while recharging
C
SS
, the soft start function inhibits PWM operation while fully
charging C
SS
to 4V to complete its cycle. Figure 4 shows this
operationwithanoverloadcondition.Notethat,inthisparticular
application, the inductor current increases to over 15A during
the C
SS
charging interval and causes an over-current trip. The
converter dissipates very little power with this method. The
measured average input power for the conditions of Figure 4 is
2.5W.
The over-current function will trip at a peak inductor current
(I
PEAK)
determined by:
where I
OCSET
is the internal OCSET current source (200
μ
A
- typical). The OC trip point varies mainly due to the
MOSFETs r
DS(ON)
variations. To avoid over-current tripping
in the normal operating load range, find the R
OCSET
resistor
from the equation above with:
1. The maximum r
DS(ON)
at the highest junction temperature.
TIME (5ms/DIV.)
SOFT-START
(1V/DIV.)
0V
0V
t
1
t
2
t
3
OUTPUT
VOLTAGE
(1V/DIV.)
FIGURE 3. SOFT-START INTERVAL
O
S
0A
0V
TIME (20ms/DIV.)
5A
10A
15A
2V
4V
FIGURE 4. OVER-CURRENT OPERATION
I
PEAK
I
---------------------------------------------------
R
DS ON
)
=
ISL6525
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